In fuel cell power plants the fuel cells generate heat which must be removed. This is accomplished by circulating coolant, usually water, through the fuel cell and through a heat rejection system.
Optimum operation of the fuel cells is accomplished at a particular temperature level which will vary on load at the plant. Accordingly, this heat rejection loop is controlled to maintain the inlet temperature to the fuel cell at a level which provides the appropriate operating temperature.
With only the fuel cell of concern this heat is rejected to waste. However, to achieve improved efficiency of the power plant it is preferable to utilize at least a portion of this waste heat which would otherwise be rejected. It is normally preferable to use this heat at the highest temperature level possible, and the waste heat system is designed with this in mind over the load range.
A heat exchanger is provided to transfer heat from the recirculating coolant to the waste heat fluid. The waste heat system is designed to operate at some particular temperature, and the heat exchanger is controlled to vary the heat exchange through the heat exchanger and maintain this temperature.
In order that proper control of the return fluid to the fuel cell can be achieved, a second heat rejection heat exchanger is provided which rejects additional heat as required to maintain the desired inlet temperature to the fuel cell. This heat rejection heat exchanger is controlled to decrease its effectiveness, usually by bypassing the flow, to effect this control.
It may happen at times that the waste heat heat exchanger in achieving its own needs decreases the recirculating coolant temperature to a level such that even with the heat rejection heat exchanger fully bypassed, the temperature to the fuel cell is too low. In this case it would be preferable to sacrifice the performance of the waste heating system and maintain the fuel cell at its desired operating temperature.